1. Field of the Invention
The invention is generally related to a low-cost, safe, and effective method for forming crack-free, conformal coatings and/or fibers of chitin.
2. Description of the Prior Art
Chitin is an unbranched .beta.(1.fwdarw.4) linked polymer of 2-acetamido-2-deoxy-D-glucose (N-acetyl-D-glucosamine) and has the following chemical structure: ##STR1##
Chitin is a material used principally by insects and marine animals for structural purposes. Chitin is typically obtained from marine waste products such as the shells of crab, shrimp, prawn, and krill, but can be obtained from a wide variety of other sources including arthropod exoskeletons and the cell walls of different forms of fungi. Chitin is insoluble in water and most commonly used solvents. The insolubility of chitin makes it difficult to process and makes formation of fibers, films, and other products therefrom problematical using prior art processing methodologies.
Consequently, more common is the reshaping and regenerating of chitosan, the acid-soluble deacetylation product of chitin. Chitosan regeneration always involves treatment with alkali so as to neutralize carboxylic acid anions, and this produces chitosan which continues to be soluble in mild acids. Chitin, by contrast, is insoluble in all common solvents, with the exception of dimethyl acetamide anhydrous ca. 8% lithium chloride, DMAc/LiCl.
U.S. Pat. No. 2,217,823 to Thor discloses a chitin processing methodology for producing articles such as filaments, films, tubes, ribbons, threads, and the like. In essence, Thor describes a methodology whereby chitin is first converted to a chitin xanthate using a caustic alkali solution and carbon disulfide. The chitin xanthate is a viscous dispersion that is subsequently processed by extrusion into a coagulating bath, such as sulphuric acid and ammonium sulphate. The chitin is regenerated in the coagulating bath and assumes a gel-like consistency. The articles are then produced by various drying operations. Thor also makes provisions for combining chitin and cellulose materials. However, the Thor methodology suffers from several drawbacks. First, caustic alkali solutions and carbon disulfide must be utilized, and this presents severe environmental, health and safety problems and increases the cost of processing vessels which must withstand the harsh environment. Second, the treatment of chitin with alkali promotes the partial conversion of chitin to chitosan, which becomes soluble in dilute acid if sufficiently deacetylated. Third, the Thor process requires the application of dissolution, filtration and regeneration technology that is being abandoned by the rayon fiber industry for environmental reasons and reasons of process complexity. This makes it improbable that the fabrication of suitable chitin articles by the xanthate method will be practical on an industrial scale.
U.S. Pat. No. 4,029,727 to Austin describes the formation of high strength chitin films and fibers in a four step process. First, chitin is dissolved with a blend of dichloroacetic acid and an anhydrous organic solvent. Second, chitin is coagulated using an excess amount of an organic liquid that is a non-solvent for chitin. Third, the coagulated article is neutralized with an alkaline reagent. This may again promote chitin deacetylation with formation of acid-soluble chitosan if conditions are sufficiently harsh. Fourth, cold drawing is used to orient the chitin fibrils. The Austin methodology is not environmentally sound since it requires the use of organic reagents which present disposal and other problems. In addition, like Thor, Austin requires multiple reagents to be used in the methodology.
U.S. Pat. No. 5,021,207 to DeLucca et al. discloses the formation of chitin acetate fibers. DeLucca is similar to U.S. Pat. No. 4,029,727 to Austin in that organic liquids are utilized to dissolve or disperse the chitin, to enable processing.
Chitosan is the product of deacetylation of chitin. It is an amorphous solid which is more soluble in water, having a pH below 6, than chitin, but it usually requires the use of aqueous organic acids to attain solubility. Chitosan is of nearly identical structure to chitin, except that it is de-acetylated. The chemical structure of chitosan is as follows: ##STR2## Because chitosan is more easily solubilized and processable, a great deal of researchers and industrialists have experimented with and/or used chitosan coatings in a wide variety of applications.
U.S. Pat. No. 4,309,534 to Austin describes a process for preparing renatured chitosan having particular optical rotation characteristics. Chitosan is aged in aqueous acid for a period of weeks. During this time period a chitosan partial acetate is formed, and the salt slowly loses the acetic acid during aging. Thereafter, the chitosan/partially deacetylated chitin is poured out as a film and immersed in dilute base, such as sodium bicarbonate, to neutralize the acetic acid and regenerate the chitosan film as the free base.
U.S. Pat. No. 5,283,064 to Suzuki et al. dislcoses the formation of chitosan coatings on medicament capsules. Chitosan is dissolved in acetic acid and is applied to medicament in a mold. The acetic acid is neutralized by inserting the mold in dilute sodium hydroxide. The neutralization step regenerates the chitosan film, and the film is subsequently dried.
Various researchers have discussed blending chitosan with cellulose to produce biodegradable films (See, for example, Isogai et al., Carb. Poly., 19:25-28 (1992), Hosokawa et al., Ind. Eng. Chem. Res. 29:800-805 (1990), Hosokawa et al., Ind. Eng. Chem. Res. 30:788-792 (1991), and Hasegawa et al., J. Appl. Poly. Sci. 45:1873-1879 (1992)).
Chitosan films are not advantageously employed in many circumstances. Unlike chitin, chitosan is a soluble entity (e.g., it can be solubilized in mild acids, especially acetic and other carboxylic acids, but also to a limited extent in HCl and HNO.sub.3, if the pH is maintained at about 4.0-6.0. Chitosan is not soluble at pHs above 7.0). Thus, the integrity of chitosan films and fibers is more easily compromised. What is needed is an improved method for preparing chitin films and fibers. The possible applications of chitin films and fibers are almost limitless.
As is discussed in Hirano et al., Science News Vol.144 page 74, chitin may have applications in chromatography, photosensitive materials, biomedical fibers and films, immunoadhesives, drug carrier and delivery mechanisms, dopants in dyes, and in antithrombogenic materials.